There is a continuing need in the field of engineering plastics for polymeric materials having good physical and mechanical properties at high temperatures, i.e., at about 200.degree. C. and higher. In addition, thermoplastic rather than thermoset polymers are preferred. The latter requirement presents formidable problems since materials which are tailored to provide satisfactory high temperature properties more often than not also exhibit poor processing properties. Thus, although they are linear polymers and by definition thermoplastic, their rheology is such that they cannot be readily processed by thermoplastic molding methods.
It is known in the prior art that incorporation of polysiloxanes into polymeric compositions changes their properties. U.S. Pat. No. 4,395,527 (Berger), for example, qualitatively discloses that the concentration of siloxane in a polyimide affects the solubility, flexibility, elongation, impact resistance and UV resistance of the polyimide. In addition polysiloxanes lower the glass transition temperature (Tg) which facilitates processing and fabrication of high molecular weight materials. This reference prefers siloxane concentrations of 40 mole % or higher and does not discuss solvent resistance nor the effect of the nature of the polyimide on this property. No particular species of polyimide is given as preferred. There is no mention of the level of polysiloxane required to be added to a polyimide molecule to achieve polymer processability without detracting from solvent resistance. Berger does teach that high ratios of polysiloxane are preferred to impart solubility, the opposite of solvent resistance. However there is no teaching or suggestion that within the narrow limits of polysiloxane content of the polyimides defined in this invention that one can retain the desirable mechanical properties and particularly the excellent solvent resistance of the unmodified polyimidesulfone and polyimideketone resins while imparting processability to them sufficient to provide commercially acceptable molding compositions.
Based on a study of the prior art, it would be expected that adding a polysiloxane to a polyimide structure effects a reduction in the polyimide mechanical properties, such as, tensile strength and tensile modulus and an increase in the polyimide solubility in organic solvents.
A case in point is the polyimidesulfones disclosed in U.S. Pat. No. 4,489,027 (T. L. St. Clair et. al.) as the resultant product of equimolar quantities of 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride and 3,3'-diaminodiphenylsulfone. These polyimidesulfones were designed to combine the processibility of the polysulfones with the solvent resistance of the polyimides.
The polyimidesulfones represented by the generic structure: ##STR1## wherein n is an integer and represents up to several hundred repeating units, have extremely good mechanical properties, evinced for example by a modulus of 600,000 psi and a tensile strength of 10,000 psi, excellent thermal resistance and a Tg of 275.degree. C. Despite the fact that they are amorphous, these materials exhibit unusually good resistance to solvents. Although they constitute an improvement in processability over that of the polyimides made from pyromellitic dianhydride and 4,4'-oxydianiline, processing problems still exist as disclosed by J. F. Dezern et. al. in NASA Technical Memorandum 86358 (1985) in terms of solvent retention and foaming. It has also been found that these polyimidesulfones are difficult to mold and prone to void production in the products or articles molded from them.
It is therefore an object of this invention to provide polysiloxane-modified polyimidesulfones and polyimideketones which exhibit good mechanical properties and solvent resistance and are readily processable.
It is another object to provide polysiloxane-modified polyimidesulfones and polyimideketones that can be molded into products free of voids.
It is also an object to provide polysiloxane-modified polyimidesulfones and polyimideketones which can be transfer molded into thin precision parts.
It is a further object to provide filled polysiloxane-modified polyimidesulfone and polyimideketone composites.
Other objects will become apparent to those skilled in the art upon a further reading of the specification.